1. Field of the Invention
The present invention relates to an organic electro luminescence device, and more particularly, to structures of a gate electrode and a source/drain electrode of a drive thin film transistor (TFT) in a top emission type organic electro luminescence device.
2. Discussion of the Related Art
In general, an organic electro luminescence device, which also is referred to as an organic light emitting diode (OLED) device, is a self-emission flat panel display device and includes a plurality of pixels and an organic light emitting diode in each of the pixels. Each of the organic light emitting diodes emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Accordingly, the organic electro luminescence does not require an additional light source and has a light weight, thin profile, and compact size.
In addition, the organic electro luminescence device generally is manufactured using a relatively simple process including a deposition process and an encapsulation process. Thus, an organic electro luminescence device has a low production cost. Further, the organic electro luminescence device can operate using a low DC voltage, thereby having low power consumption and fast response time. The organic electro luminescence device also has a wide viewing angle, and high image contrast. Moreover, the organic electro luminescence device is an integrated device. Thus, the organic electro luminescence device has high endurance from external impacts and a wide range of applications.
A passive matrix type electro luminescence device that does not have a switching element has been widely used. In the passive matrix type electro luminescence device, scan lines intersect signal lines to define a matrix shape device, and the scan lines are sequentially driven to excite each pixel. However, to achieve a required mean luminescence, a moment luminance needs to be as high as the luminance obtained by multiplying the mean luminescence by the number of lines.
There also exists an active matrix type electro luminescence device, which includes thin film transistors as switching elements within each pixel. The voltage applied to the pixels are charged in a storage capacitor Cst so that the voltage can be applied until a next frame signal is applied, thereby continuously driving the organic electro luminescence device regardless of the number of gate lines until a picture of images is finished. Accordingly, the active matrix type electro luminescence device provides uniform luminescent, even when a low current is applied.
FIG. 1 is a circuit diagram of a pixel structure of an active matrix type organic electro luminescence device according to the related art. In FIG. 1, gate lines 2 are formed along a first direction and data and power lines 3 and 4 are formed along a second direction intersecting the gate lines 2, thereby defining a plurality of sub-pixel regions. In addition, a switching TFT 5 is formed in each of the sub-pixel regions. A storage capacity Cst 6 is connected to the switching TFT 5 and the power line 4. A drive TFT 7 that is a current source element is connected to the storage capacitor Cst 6 and the power line 4.
An organic electro luminescence diode 8 is connected to the drive TFT 7. When a current is applied to an organic light emitting material of the organic electro luminescence diode 8 in a forward direction, electrons and holes are recombined, moving through a P-N junction between an anode electrode as a hole donor and a cathode electrode as an electron donor. Therefore, the energy of the organic electro luminescence diode 8 becomes lower, thereby generating an energy difference and causing light emission.
The organic electro luminescence device may be classified into a top emission type and a bottom emission type based on its light emission direction, and FIG. 2 is a schematic sectional view of a bottom emission type organic electro luminescence device according to the related art. In FIG. 2, an organic electro luminescence device 10 includes a first transparent substrate 12. A TFT array 14, a first electrode 16, an organic luminescent layer 18 and a second electrode 20 are formed on the first substrate 12. The organic luminescent layer 18 includes red, green and blue organic luminescent material R, G, B to corresponding to each of sub-pixels in a pixel P.
The first substrate 12 is adhered to a second substrate 28, on which an absorbent 22 is formed, by a sealant 26, thereby completing the organic electro luminescence device that is encapsulated. The absorbent 22 is for removing moisture and oxygen that may be infiltrated into the encapsulated organic electro luminescence device. In particular, a portion 25 of the second substrate 28 is etched and the absorbent 22 is filled in the etched portion and fixed by a tape. The first and second substrates 12 and 28 are formed of a transparent insulating material such as glass or plastic.
FIG. 3 is a sub-pixel of a TFT array in the organic electro luminescence devices shown in FIGS. 1 and 2. In FIG. 3, gate lines 32 and data lines 34 intersect each other with an insulating layer formed therebetween and define a pixel region P. In addition, power lines 35 are formed in parallel with the data lines 34. A switching TFT TS is formed in the pixel region, and is connected to a respective gate line 32 and a respective data line 34. The switching TFT Ts includes a gate electrode 36, an active layer (not shown), a source electrode 46, and a drain electrode 50. The gate electrode 36 of the switching TFT TS is connected to the gate line 32 and the source electrode 46 is connected to the data line 34. The drain electrode 50 is connected to a gate electrode 38 of a drive TFT TD through a first contact hole 54. A source electrode 48 of the drive TFT TD is connected to the power line 35 and a drain electrode 52 of the drive TFT TD is connected to a first electrode 16 through a second contact hole 56.
The TFT provided on the pixel forms an active layer using amorphous silicon. Since the mobility of the amorphous silicon is lower than that of crystal silicon by 0.5-1 cm2/Vsec, a ratio of a width to a length W/L of the drive TFT TD needs be increased to drive the organic luminescent layer. The width of the drive TFT TD is an overlapped portion between the active layer and the source/drain electrode on the gate electrode 38. The length of the drive TFT TD is a distance between the source and drain electrodes 48 and 52. In order to increase the ratio W/L, the size of the drive TFT TD also increases.
However, when the size of the drive TFT TD is increased, the aperture rate of the bottom emission type organic electro luminescence device is reduced. As a result, current density applied to the organic electro luminescence layer is relatively high, and the drive TFT TD experiences DC stress for many hours, thereby reducing the life span of the organic electro luminescence device.